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60 seconds
each stop is twice as long as the previous
prizes for guessing what the next stop is 
ah
well that's about 1000 times the unfiltered exposure
2 to the power 10 = 1024
If you have a smart phone get the "long exposure" app it works it all out for you.
But basically Each stop doubles the time of the last exposure, so 60,120,240,480,960 etc (Your maths is wrong Ade!)
A 30 second exposure without filter would be 8 hours 32 minutes with !!!
0.3seconds gives a 30 second exposure.
Wrong maths???
10 stops as follows Nick - each step being the factor you multiply by to get the new exposure
2
4
8
16
32
64
128
256
512
1024
so 0.3 seconds is approx 1000*0.3 = 300 seconds
30 seconds is approx 1000*30 = 30000 seconds
When you've worked out you're maths is incorrect Nick, let me know
Quote: In your first reply you said 1024 seconds
where?
I seem to recall saying this...
Quote: ah
well that's about 1000 times the unfiltered exposure
2 to the power 10 = 1024
no mention of seconds..... 
errrrm
Actually the only difference is that Ade said 'about 1000 times'. The difference between your answers is only down to multiplying the difference between 1000 and 1024.
Whether a 2.4% difference in exposure is noticeable is something I will leave to those with a 10 stop filter. 
Ian
It's still a bloody long time !!
Make sure that you've got a good tripod !!!!!!
Can't believe this is still going.
Think we will call it a draw
on a semi serious note it's when you understand "stops" you can start thinking on your feet, be it shutter speed , aperture or the one people tend to forget about ISO.
Get the calculator ready Ade for round two
ian in the litreture it says the 10 stopper from Lee says it can vary between 9 and 11 stops which in time terms can be quite a bit. But +\- 1 stop is easily controlled in RAW.
No reciprocity issue but you do have random noise and photodiode leakage with most sensors. - Rendering anything over a certain length a noisy mess.
This is why people "in the know" will burst multiple digital images for long exposures and not use bulb. (Since the images can later be stacked by value in software)
And if anyone disagrees go use your digital camera on a 4 hour bulb then 240 stacked 1 min exposures.
Obviously you will then have to take into account if your subject is a point source or not, and change format accordingly.
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